To comply with safety regulations and to monitor the inclination of well boreholes, among other reasons, the hole may be surveyed periodically during drilling. It is important, for example, that the location of the drill bit relative to the mouth of the hole is known so that a relief well can be drilled in the event of a blow-out.
It is presently known to measure the inclination of a drilled hole using one of four types of devices. The first type of device is a drift indicator, the second is a magnetic single shot device, the third is a mechanical measuring-while-drilling device (MMWD), and the fourth is a directional measuring-while-drilling device (DMWD).
The first two types of device (the drift indicator and the magnetic single shot device) have been used for more than 50 years. They require a person drilling a well to lower the device into the hole, wait for the device to perform a reading, raise the device from the hole, and then check the measurement taken by the device. Frequently, a second measurement is required to confirm the accuracy of the first measurement. These devices are very expensive to use because the drilling procedure is halted while the device is being used to survey the hole.
The third type of device (the MMWD) has been used for more than 40 years. It is located above the drill bit in a purpose-built collar. This device uses a swinging mechanical pendulum to measure the inclination of the device with reference to the vertical plane. This inclination reading is linked to a mechanically activated plunger which, when activated, produces a pulse which is transferred to the surface. Each pulse represents 0.5 degrees of inclination. This provides a measurement of the verticality (the downhole inclination) of the hole.
The fourth type of device (the DMWD) is similar to the MMWD but conveys information about the inclination of the hole by means of binary code rather than by mechanically activated pressure pulses. At the drilling console, the code is received, decoded and the results are displayed to the drill operators. The DMWD has a number of disadvantages associated with it. For example, it usually needs at least one trained engineer to operate it correctly and it is more expensive than the other devices.
Presently, the most commonly used device is the MMWD device. It is relatively inexpensive to run and does not require an additional trained engineer to operate it. However, these devices are not very accurate or reliable. They are also very expensive to make because they are housed in collars which can cost more than the combined cost of the component parts inside them. A further disadvantage of these devices is that they are sometimes lost downhole, that is, they have to be abandoned, for example, in situations where the bottom hole assembly becomes stuck.
In the oil and gas exploration and production industry, a wide range of downhole tools are used for performing specific functions in the downhole environment. Many of these tools are fluid pressure activated and include relatively complex flow control mechanisms for controlling activation of the tool. Frequently these tools require a positive fluid flow for activation, for example, flow past the tool when located in a borehole.
Other tools, such as centralisers which are used for centralising a secondary tool in tubing in a well borehole, are mechanical and may include, for example, fins such as rubber fins or sprung arms. Where rubber fins are used, the fins are dimensioned to be a close fit within a tubular in which the centraliser is located whilst sprung arms are compressed inwardly on location of the centraliser within the tubular. In both cases, this acts to centralise a body of the centraliser and thus a tool coupled to the centraliser, such as a drill bit, within the tubular. However, fixed dimension centralisers such as these create potential problems when subsequently removed from the borehole, as a tool such as a packer, valve or jar may have been located in the borehole above the centraliser, these tools restricting the diameter of the borehole and making it difficult to withdraw the centraliser.
It is amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.